Polyoxyalkylene alkyl ether carboxylic acid and salts thereof are compounds produced by substituting polyoxyalkylene alkyl ether with a carboxyl group at the terminal end of the ether, exhibit high degrees of foaming and emulsification, and are known as surfactants used in cosmetics, emulsifiers, solubilizers, dispersants, gelling agents, and detergent bases, for example. Properties of polyoxyalkylene alkyl ether carboxylic acid and salts thereof can be modified by changing pH. Polyoxyalkylene alkyl ether carboxylic acid and salts thereof have excellent stability against hard water, and aqueous solutions thereof are stable to various polyvalent metal ions such as aluminium ions, are gentle to the skin, and have small inhibitory effects on enzymes. Polyoxyalkylene alkyl ether carboxylic acid and salts thereof are thus expected to have various applications.
There have been various known methods for producing polyoxyalkylene alkyl ether carboxylic acid or a salt thereof. One of such methods is a method in which polyoxyalkylene alkyl ether is oxidized with oxygen in the presence of a catalyst in which a noble metal is supported (see, for example, Patent Document 1).
Patent Documents 2 and 3 describe methods for producing salts of polyoxyalkylene alkyl ether carboxylic acid by supplying oxygen to a suspension containing polyoxyalkylene alkyl ether, a noble metal catalyst, and an alkaline material to cause catalytic oxidation of polyoxyalkylene alkyl ether. In these methods, an oxygen supply starts after the amount of dissolved oxygen in the suspension has been set at 0-1 ppm, and then the amount of dissolved oxygen in the suspension is kept in the range more than 0 ppm and less than or equal to 1 ppm.
Patent Document 4 shows a method for producing a salt of polyoxyalkylene alkyl ether carboxylic acid by using a continuous stirred tank reactor.